The present invention relates to a hydraulic brake circuit for a motor vehicle and more particularly to such a circuit equipped with an anti-lock device and with an anti-skid device for the wheels of the vehicle.
The locking of the wheels is a phenomenon which occurs during braking, when the brake force exerted on a wheel causes the wheel to slip on the road. The word "skidding" is used when, in the absence of any braking, a driving wheel slips on the road because the engine torque driving it exceeds the reaction torque attributable to the friction at the wheel/road interface. In both cases, the steering of the vehicle becomes difficult and even impossible to control because of the loss of grip of the wheels.
To guard against this dangerous situation, devices were designed which detect the imminent occurrence of the locking of a wheel and which, in response, correct the pressure of the brake fluid in the wheel brake by means of successive cycles of expansion and compression, until the risk of slipping of the wheel has disappeared.
Devices were also designed which detect imminent skidding and, in response, brake the wheel in such a way as to reduce the resulting engine torque received by the wheel and thus prevent the latter from slipping on the road.
U.S. Pat. No. 4,944,565 which is now U.S. Pat. No. 4,964,565, assigned to the present assignee and entitled "Hydraulic brake circuit for a motor vehicle equipped with wheel anti-lock and anti-skid devices", describes a brake circuit which comprises a master cylinder controlled by a solenoid relief valve, by means of which the master cylinder normally communicates with the brake, an actuation of this solenoid valve under the control of the anti-lock device putting the brake in communication with brake-fluid storage means, and a pump, of which an actuation controlled by the anti-lock device causes the brake fluid to be delivered from the storage means to the brake, via the solenoid relief valve, when the latter is not actuated. The circuit also comprises a solenoid anti-skid valve which is placed upstream of the solenoid relief valve and by means of which the master cylinder normally communicates with the brake, an actuation of the solenoid anti-skid valve under the control of the anti-skid device putting the master cylinder in communication with a suction port of the pump, the actuation of which is likewise controlled by the anti-skid device.
When the wheel anti-skid device detects the imminent slipping of a wheel, the solenoid anti-skid valve is energized and the pump is actuated. The master cylinder then communicates with the brakes of the vehicle via the pump. On the assumption that the brake pedal is not actuated, the brake fluid contained in the reservoir of the master cylinder is then sucked up by the pump and delivered to the brakes, so that an action to brake the wheels is exerted automatically until the risk of slipping of the latter has disappeared.
In the described use for an "X"-type brake circuit with two independent circuits, each controlling a front-wheel brake and a rear-wheel brake respectively, it is clear that the solenoid anti-skid valve of the abovedescribed circuit is placed upstream of the feed lines of the two brakes. In the anti-skid phase, therefore, no action on the two brakes is possible from the brake pedal.
However, the driver may have to actuate this if the solenoid anti-skid valve has been triggered accidentally or even if the latter is actuated normally in order to prevent imminent skidding. Action on the brake pedal may in any case be justified for safety reasons, in order to brake a non-driving wheel while the driving wheel is assumed to be braked by means of the anti-skid device.
Under these two circumstances, with the circuit of the abovementioned patent application, the braking action on the non-driving wheel will be delayed considerably because of an absorption of brake fluid by the storage means of this fluid which are put in communication with the master cylinder during an anti-skid period.